1. Field of the Invention
The present invention relates to an improved deep hole boring head and a deep hole boring method for boring a production piece, and more particularly a production piece made of titanium.
2. Discussion of the Background
Deep hole boring differs from shallow hole boring, or from drilling, by the L/D ratio between the length L and the diameter D of the hole to be made. Thus, deep hole boring in the present specification means a boring method used to make holes having an L/D ratio greater than or equal to 10. With such an L/D ratio, it is particularly necessary to clear away the chips cut from the production piece during boring.
Also, it will be noted that the invention is more particularly intended for boring large diameter holes, usually greater than 40 mm and, preferably, greater than or equal to 65 mm.
Deep hole boring tools are already used for boring steel alloy pieces, such as 300M and 4340M alloy steels. These tools are formed of a rotary drive shaft, at the front end of which is mounted a deep hole boring head which comprises a body whose front face has at least one cutting edge followed by a recess which leads to a duct inside the said body, this duct allowing the chips formed during boring to be cleared away rearwards.
To make it easier to clear away the chips, limit the heating and reduce the friction between the boring head and the bottom of the hole made, a boring liquid, usually undiluted oil or oil that is soluble between 10 and 15%, is used.
The general shapes of the front and rear parts of the boring head have a symmetry of revolution relative to the central axis about which the head and the tool are rotated.
The rear part of the boring head and the drive shaft are hollow cylinders, the diameter of the drive shaft being less than or equal to the diameter of the rear part of the boring head. The latter diameter is furthermore less than the maximum diameter of the front part of the boring head, so that, during boring, there remains an annular space between the rear part of the boring head, the drive shaft, and the walls of the hole bored. This annular space, necessary for limiting the friction between the tool and the walls of the hole, is also used for bringing the boring liquid to the front part of the boring head. The liquid chip-laden is then cleared away via the internal duct passing through the rear part of the boring head and the drive shaft.
Because of this annular space, it is necessary to guide the boring head when it moves forward. For this purpose, it is a known practice to use three guide pads usually coated with carbide or Celoron (registered trademark) pads, that are evenly distributed and protrude from the outer lateral periphery of the rear part of the head, and that are intended to press against the walls of the hole formed during boring.
However, the boring heads known to date are unsuitable for boring titanium (Ti) pieces. Specifically, the titanium adheres to the guide pads, which causes the boring head to seize and it then becomes virtually impossible with the machines currently used in the boring field to develop a sufficiently high boring torque to continue rotating the head. Furthermore, premature wear of the guide pads is noted, which necessitates frequent replacement of the pads, while their price is high due to their specialist nature and the materials they are made of. Finally, because of these unsatisfactory boring conditions, the geometry of the hole made is so bad (in particular, significant deviations are observed relative to the boring axis) that it is very difficult, even impossible, to rectify it by machining. In addition, this subsequent machining step is made difficult by the presence of the carbide particles originating from the guide pads.